There are many scientific studies in tropical, temperate and polar waters which depend on or can be materially assisted by a means to locate and identify various species of cetaceans. There are also commercial reasons to locate these animals.
Location can be done by triangulation using the angle of arrival measured from widely spaced vessels or other acoustic sensors whose position is known. Multiple sensors or vessels are expensive and the location computations are complex so that correlation and location can take hours or even weeks. This prohibits adaptive placement of resources to improve data or resolve ambiguities.
It is known that yellowfin tuna follow dolphins in certain areas of the ocean, particularly the Eastern Tropical Pacific Ocean, a geographically well defined area. This is an area of large, harvestable schools of tuna. A significant factor is that in the past, the dolphins around which the tuna tended to gather or follow, were caught in nets along with the target tuna and either injured or killed in many instances. This waste of mammal life has become socially undesirable and, in accordance with the International Dolphin Conservation Act of 1992 of the United States of America, yellowfin tuna must be harvested by techniques which do not harass, harm or kill the associated dolphins. In view of the fact that this law has effect from 1 Mar. 1994, the setting of nets on dolphins in order to catch tuna which follow the dolphins is prohibited. The law incorporates harsh embargo provisions, making it economically unattractive for nations to sell any fish or shellfish products to the United States which use nets to harvest tuna that follow dolphins. Thus techniques for harvesting tuna, particularly yellowfin tuna that follow dolphins is the Eastern Tropical Pacific Ocean, must necessarily be accomplished by means other than setting nets on dolphin.
However, there is no reason to restrict the use of dolphins as a means of locating schools of tuna. Dolphins emit identifiable calls which can be an acoustic basis for finding tuna for harvesting purposes.
Examples of underwater acoustic detection system are shown in U.S. Pat. Nos. 4,622,657 (Clare); 4,509,151 (Anderson); and 3,893,061 (Slawsky et al.).
The Clare patent uses an array of hydrophones arranged in linear groups at right angles to each other or, alternatively, hydrophones located at comers of a regular pentagon. One important aspect of the disclosure of this reference is its unequal spacing between detecting pairs. Successive zero-crossings of waves detected by the two hydrophones in each of several different pairs is used to determine direction or beating. The system of the Clare patent appears to use analog signals fed to a computer where the time delay of the arriving signal is computed between pairs of hydrophones. However, this patent does not show any bearing solution output device or indicator. Further, no aspect of the Clare patent relates to range determination through the use of the hydrophone array.
The Anderson patent has as its primary purpose the detection and classification as to type and location of different marine animals using a passive towed array of hydrophones. There is no attempt in Anderson to determine range, and direction is determined only in a general, relatively crude fashion. This mechanically derived bearing to a sound source is determined by steering the towing vessel and determining increases or decreases in the levels of signals received. Anderson's bearing solution is based on the ability of observers to listen to the output of the sensors and binaurally determine the general bearing to the sound source. This patent discloses no graphic output display and no digital storage mechanism relating to bearing solutions. Anderson's system is able to determine different incoming frequencies only by mechanically changing the interhydrophone distances of each group of sensors. Further, by employing a towed array, the Anderson system is impractical for commercial fishing operations.
A third underwater passive acoustic detection system is shown in the Slawsky patent. This patent relates to a harbor entrance or other defined body of water where the system of the patent is designed to detect approaching ships or other moving objects. Pursuant to a signal of higher DC level than ambient signals detected by the hydrophone, a signal is transmitted underwater to a central receiving hydrophone where appropriate indications are made. There is no attempt by Slawsky to determine range or bearing by this means. Further, it is not directed to marine animals.
For many years there has been interest in locating objects, primarily animals, for environmental studies of the impact of human activity on other species. One common method with land animals has been to attach radio telemetry equipment to individual animals and track the movement of the animals using radio detection equipment. The same technique has been applied in the ocean with regard to radio tracking animals that come to the surface to breathe, such as, whales, dolphins and turtles. In addition, investigators have attached sonic pingers to fish and sharks to track them as they move through the water. All these techniques require the attachment of a tracking device to the animal to be tracked. Because small toothed cetaceans, including dolphins, are extremely sensitive to sounds in the ocean, and use sound as one of their primary sensory modalities, it would be inappropriate, and quite likely illegal, for an acoustic location system to use any form of active or active/passive acoustic system, such as sonar or acoustic tags, to locate the dolphins. This is because such a system would have a high potential for disrupting the behavior and biology of the dolphins and possibly causing them harm.